Jongchul Lim

A diketopyrrolopyrrole-containing hole transporting conjugated polymer for use in efficient stable organic–inorganic hybrid solar cells based on a perovskite

Poly[2,5-bis(2-decyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2′-bithiophen-5-yl) ethene] (PDPPDBTE) was successfully incorporated as a p-type hole transporting material in solid-state organic–inorganic hybrid solar cells. The excellent optical and electrical properties of organo-lead halide perovskite (CH3NH3PbI3) nanocrystals used as light harvesters yielded a 9.2% power conversion efficiency (PCE) for the best-performing cell that exceeded the value (7.6%) obtained from the best hole conductor yet reported (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene, spiro-MeOTAD). The high PCE was attributed to the optimal oxidation potential (5.4 eV) and excellent charge carrier mobility of the polymer. The hydrophobicity of the polymer prevented water permeation into the porous perovskite heterojunction, and long-term aging tests over 1000 hours confirmed the enhanced stability of the PDPPDBTE-based cells.

Charge Density Dependent Mobility of Organic Hole‐Transporters and Mesoporous TiO2 Determined by Transient Mobility Spectroscopy: Implications to Dye‐Sensitized and Organic Solar Cells

Transient mobility spectroscopy (TMS) is presented as a new tool to probe the charge carrier mobility of commonly employed organic and inorganic semiconductors over the relevant range of charge densities. The charge density dependence of the mobility of semiconductors used in hybrid and organic photovoltaics gives new insights into charge transport phenomena in solid state dye sensitized solar cells.

Sulfur-incorporated carbon quantum dots with a strong long-wavelength absorption band

In this work, we synthesize sulfur-incorporated carbon quantum dots (S-CQDs) and report the effect of sulfur on their electronic structure. Sulfur provides the density of states or emissive trap states for photoexcited electrons, and hence improves absorbance and photoluminescence intensity in the long-wavelength (∼500 nm) regime. The formation of the emissive trap states in the band-gap is confirmed by time-resolved emission decay spectroscopy. It is revealed that the emissive trap states prolong the fluorescence lifetime of low energy (∼2.5 eV) photoexcited electrons. To explore further the change in the band-gap energy, we demonstrate charge transport in S-CQD films that serve as the channels of field-effect transistors (FETs). The turn-on voltage of the S-CQD-based FETs decreases with the increase of the sulfur concentration, which is consistent with the optical changes. Our results establish a technical basis to incorporate heterogeneous atoms into CQDs and examine the related changes made to their optoelectronic properties. This method would open up new prospects to control the band-gap energy of CQDs in mild conditions, and hence promote their applications in imaging agents and optoelectronic devices.

A novel quasi-solid state dye-sensitized solar cell fabricated using a multifunctional network polymer membrane electrolyte

A series of liquid junction dye-sensitized solar cells (DSCs) was fabricated based on polymer membrane-encapsulated dye-sensitized TiO2 nanoparticles, prepared using a surface-induced cross-linking polymerization reaction, to investigate the dependence of the solar cell performance on the encapsulating membrane layer thickness. The ion conductivity decreased as the membrane thickness increased; however, the long term-stability of the devices improved with increasing membrane thickness. Nanoparticles encapsulated in a thick membrane (ca. 37 nm), obtained using a 90 min polymerization time, exhibited excellent pore filling among TiO2 particles. This nanoparticle layer was used to fabricate a thin-layered, quasi-solid state DSC. The thick membrane prevented short-circuit paths from forming between the counter and the TiO2 electrode, thereby reducing the minimum necessary electrode separation distance. The quasi-solid state DSC yielded a high power conversion efficiency (7.6 → 8.1%) and excellent stability during heating at 65 °C over 30 days. These performance characteristics were superior to those obtained from a conventional DSC (7.5 → 3.5%) prepared using a TiO2 active layer with the same thickness. The reduced electrode separation distance shortened the charge transport pathways, which compensated for the reduced ion conductivity in the polymer gel electrolyte. Excellent pore filling on the TiO2 particles minimized the exposure of the dye to the liquid and reduced dye detachment.

Thermodynamic Control over the Competitive Anchoring of N719 Dye on Nanocrystalline TiO2 for Improving Photoinduced Electron Generation

TiO(2) electrodes, sensitized with the N719 dye at high immersion temperatures during the sensitization process, were found to have large fractions of weakly bound N719 on the electrode surface, which resulted in dye aggregation and decreased device longevity. These disadvantages were ameliorated using a low-temperature stearic acid (SA)-assisted anchoring method described here. The activation energy (ΔE(NS)(++)) and relative fraction of strongly bound N719 were twice as large as the respective values obtained without the use of SA. Slowing of adsorption, both by thermal means and through SA-mediated processes, effectively controlled the binding mode of N719 on the surface of TiO(2). The resulting sensitized electrodes displayed enhanced device longevity and improved generation of photoinduced electrons.

Reduced charge recombination by the formation of an interlayer using a novel dendron coadsorbent in solid-state dye-sensitized solar cells

3,4,5-Tris(dodecyloxy)benzoic acid (DOBA) and the Z907 dye were coadsorbed to form a light-sensitizing monolayer in a solid-state dye-sensitized solar cell (sDSC). Coadsorption of DOBA which has three hydrocarbon chains permitted preparation of a denser monolayer of dyes and DOBA. This dense monolayer formed interlayer between TiO2 and Spiro-OMeTAD (hole conductor), effectively preventing charge recombination, while half of the photocurrent was dissipated via recombination reaction when Z907 solely anchored on the surface of TiO2. Moreover, the DOBA induced a lower population of density-of-state (DOS) in the surface of TiO2, shifting the position of the conduction band (CB) toward negative values. This resulted in higher open-circuit voltage (VOC) for the device made with Z907 and DOBA than that of the Z907-sensitized device. These surface properties were investigated using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent/photovoltage spectroscopy (IMPS and IMVS).

Chemical compatibility between a hole conductor and organic dye enhances the photovoltaic performance of solid-state dye-sensitized solar cells

A series of organic dyes having an unsymmetrical geometry, 3-(5′-{4-[(4-tert-butyl-phenyl)-(4-fluoro-phenyl)-amino]-phenyl}-[2,2′]bithio-phenyl-5-yl)-2-cyano-acrylic acid (D-F), 3-(5′-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2′] bithiophenyl-5-yl)-2-cyano-acrylic acid (D-CH33), and 3-(5′-{4-[(4-tert-butyl-phenyl)-(4-methoxy-phenyl)-amino]-phenyl}-[2,2′]bithiophenyl-5-yl)-2-cyano-acrylic acid (D-OCH3), were designed and synthesized for use in solid-state dye-sensitized solar cells (sDSCs). The dye regeneration energy levels and surface properties were characterized to determine the hole transfer yield from the oxidized dye to the hole conductor (spiro-OMeTAD) by measuring the degree of pore-filling by the spiro-OMeTAD and the transient absorption spectra (TAS). An electrode sensitized with D-OCH3 exhibited the highest spiro-OMeTAD filling fraction and hole transfer quantum yield (Φ) to spiro-OMeTAD, resulting in an enhanced photocurrent and a power conversion efficiency of 3.56% in the sDSC, despite a lower energy driving force for hole transfer compared to those of D-F, or D-CH33. This result illustrates the importance of the chemical compatibility between the hole conductor and the dye on the surface of TiO2.

Well-defined all-conducting block copolymer bilayer hybrid nanostructure: selective positioning of lithium ions and efficient charge collection.

A block copolymerization of nonfunctionalized conducting monomers was developed to enable the successful synthesis of a highly insoluble 3,4-(ethylenedioxy)thienyl-based all-conducting block copolymer (PEDOT-b-PEDOT-TB) that could encapsulate nanocrystalline dyed TiO2 particles, resulting in the formation of an all-conducting block copolymer bilayer hybrid nanostructure (TiO2/Dye/PEDOT-b-PEDOT-TB). Lithium ions were selectively positioned on the outer PEDOT-TB surface. The distances through which the positively charged dye and PEDOT-TB(Li(+)) interacted physically or through which the TiO2 electrode and the Li(+) centers on PEDOT-TB(Li(+)) interacted ionically were precisely tuned and optimized within ca. 1 nm by controlling the thickness of the PEDOT blocking layer (the block length). The optimized structure provided efficient charge collection in an iodine-free dye-sensitized solar cell (DSC) due to negligible recombination of photoinduced electrons with cationic species and rapid charge transport, which improved the photovoltaic performance (η = 2.1 → 6.5%).

Fast cascade neutralization of an oxidized sensitizer by an in situ-generated ionic layer of I− species on a nanocrystalline TiO2 electrode

We report a novel way to accelerate the rate of oxidized sensitizer neutralization on nanocrystalline TiO2 electrode surfaces using a novel coadsorbent, 3,4,5-tris-butenyloxy benzoic acid (TD), having three terminal double bonds. 1H NMR and contact angle measurements revealed that the terminal double bonds reacted with I2 to form an in situ-generated ionic layer of I− species. Transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) studies demonstrated that I− species neighbouring the cationic dye molecules (D+) accelerate the neutralization (or regeneration) rate (kD+), as well as decrease the recombination reactions of photoinduced electrons with D+ (k1) and I3− (k2). Dye-sensitized solar cells treated with TD exhibit a power conversion efficiency of 10.2%, which is 22% higher due to the simultaneous improvements in JSC and VOC, even at 15% low dye loading levels, compared to the values obtained from a conventional device.

Controlling Nucleation and Growth of Metal Halide Perovskite Thin Films for High‐Efficiency Perovskite Solar Cells

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.